This invention is concerned with a detergent bar containing hydroquinone as a skin bleaching ingredient.
Melanin is a dark, sulfur-containing pigment normally found in the skin, hair, eyes and certain nerve cells which is produced in cells called melanocytes. Melanin is produced in the melanocytes by the conversion of the amino acid tyrosine in the presence of the enzyme tyrosinase. It is generally believed that the number of melanocytes varies widely among individuals.
There frequently occurs in a given individual a localized area of the skin wherein the malanin density within the melanocytes is markedly increased resulting in a skin color in the area affected far darker than normal background skin color. These localized areas of hyperpigmentation are commonly referred to as Brown Spots, Age Spots or Liver Spots. Women are also subject to these melanin-dense spots as a result of child bearing and taking birth control pills. The skin disfigurement which results from these areas of hyperpigmentation, which may be more or less permanent in character, is often a source of great distress to the individual.
In the past there have been described a number of topical skin compositions containing one or more ingredients capable of reducing the melanin density in the melanocytes of the skin. Such ingredients are termed depigmentation agents or bleaching agents and are absorbed into the lower layers of the skin in order to inhibit the formation of melanin in the melanocytes.
The most frequently described bleaching agents are based on hydroquinone or derivatives of hydroquinone, Benzyloxyphenol, the monobenzylether of hydroquinone, is one such hydroquinone derivative which has found wide application as a bleaching agent. In U.S. Pat. No. 3,060,097 for example, a skin bleaching composition is described comprising benzyloxyphenol, sodium hypochlorite, a penetrant and a soothing agent such as a lanolin compound and a solvent acting also as a penetrant. Unfortunately benzyloxyphenol is not metabolized to any great degree when absorbed into the skin and is associated with incidents of irreversible depigmentation simulating vitiligo (patches of depigmentation often having a hyperpigmented border and enlarging slowly). In addition benzyloxyphenol is transported by the lymph system and may cause irreversible depigmentation in areas of the body far removed from the site of application. Methoxyphenol, another ether of hydroquinone, has also been used in cosmetic compositions for depigmentation but is relatively insoluble in the aqueous media which is a constituent of many cosmetic formulations.
A substituted isomer of hydroquinone, 4-isopropyl catechol has also been employed as the active ingredient in cosmetic formulations to effect skin depigmentation (see South African Pat. Appl. 716,890). Mono and difatty acid esters of hydroquinone have also been employed in topical skin depigmentation compositions and are described in European patent Application No. 82301102.8.
Hydroquinone itself is conveniently used in cosmetics for the treatment of hyperpigmentation since it is effective, soluble in water and readily metabolized and excreted from the body. Hydroquinone, however, when present in an alkaline environment is unstable and is oxidized to the quinone form imparting an accompanying browning effect to any composition in which it is incorporated. To prevent such oxidation it is necessary to incorporate an antioxidant into the hydroquinone-containing composition such as ascorbic acid or butylated hydroxy anisole to modify the hydroquinone itself. Hydroquinone is also a skin irritant.
Alcoholic-gel cosmetic sticks such as stick deodorants offer a medium for cosmetic bleaching using hydroquinone since it is soluble in alcohol. However, the gelling agents used in such sticks are soaps such as sodium stearate and their alkalinity readily and rapidly decomposes hydroquinone.
Hydroquinone has also been stabilized by incorporating it into an anhydrous medium. In U.S. Pat. No. 4,466,955, a non-aqueous cosmetic skin preparation is described in which hydroquinone is dissolved in polypropoxylated or polyethoxylated fatty ethers and this anhydrous solution is incorporated into an extended oil and wax non-aqueous cosmetic base. In such an anhydrous oil-wax base the hydroquinone is more stable and less prone to oxidation since oxygen is less soluble in waxes than in water and oxygen from the air does not reach the wax-dissolved hydroquinone as readily as if it were solubilized in water. The polyalkoxylated agents act as solubilizing agents in the hydroquinone and are themselves cosmetic skin penetration agents which allow controlled release of hydroquinone.
The present invention, on the other hand, provides a non-cosmetic synthetic detergent bar for use on the skin containing hydroquinone as a depigmentation agent and water which is maintained at acid to neutral pH to prevent oxidation of the hydroquinone. The bar is characterized by having a long shelf life.